Amplifier



C. S. ROOT May 12, 1942.

AMPLIFIER Filed June 20, 1939 Inventor: Chaf-|ej S. ooh, 1: JV a.

y ms Attorney.

Patented May 12, 1942 AMPLH'IER Charles S. Root, Stratford, Conn., assignor to General Electric Company, a corporation of New York Application June 20, 1939, Serial No. 280,119

14 Claims.

My invention relates to amplifiers and more particularly to audio frequency amplifiers such as are commonly employed in radio receivers.

It has for one of its objects to provide certain improvements in such amplifiers wherebytheir cost is reduced and the operation is improved.

More specifically, it has for one of its objects to avoid certain difficulties in such amplifiers resulting from use in connection therewith of loud speakers of the permanent magnet type. Loud speakers for radio receivers have commonly been provided with electromagnetic field coils which are energized from the rectifier which supplies energizing currents to the receiver. These coils, of course, have high inductance and are utilized as smoothing reactors in the rectifier output system thereby to reduce the production of hum at the output of the receiver. Since permanent magnet speakers now coming largely into use have no such coil, the problem is presented of maintaining the production of hum at its former low level and at the same time retaining the reduction in cost of the receiver obtained by substitution of the permanent magnet speaker for the speaker having the electromagnetic field coil.

One of the objects of my invention is to provide means whereby this result is accomplished and whereby it may be accomplished even in receivers having large output volume and'in which the production of hum must be keptat a very low level.

Commonly degenerative circuits have been used for reduction of hum. One of the objects of my invention is to effect certain improvements in such circuits whereby the degeneration voltage utilized may be increased and at the same time prevent undesired regeneration at both high and low audio frequencies in the voice range.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure represents an embodiment of my invention.

Referring to the drawing, I have shown therein a two-stage amplifier comprising electron discharge devices I and 2. The anode of the electron discharge device I is coupled to the grid of the electron discharge device 2 by means of a coupling condenser 3. The anode of discharge device 2 is connected through the primary winding of an output transformer 4 to the positive side +B of the source of operating current (not shown). This point +B is also connected through a resistance 6 to the screen grid I of the electron discharge device 2 and through resistance 8 to the anode of electron discharge device I. The output of transformer 4 may be connected to the voice coil of a loud speaker or other suitable signal responsive device.

In the form of the invention shown the discharge device I is one having a cathode 9 which cooperates with the signal, or control, grid I0 and anode II of the amplifier, and also with an anode I2 of a diode rectifier. The circuit of this diode rectifier extends from the anode I2 through the secondary winding of input transformer l3, which may comprise the output transformer of the intermediate frequency portion of the superheterodyne receiver, load resistance I4, resistance i5, and thence to the cathode 9 of the diode. Signal currents appearing on resistance I4 are supplied to the grid I0 of the amplifier in discharge device I through coupling condenser I6 and the potentiometer II, the circuit of this potentiometer being completed through resistances I8 and I 5 to the bottom of the diode load resistor I4. The usual diode load bypass condenser I4 is connected between the upper terminal of resistance l4 and the cathode 9 of the diode. This cathode is also connected to the point 28 between resistances I8 and IS. The variable tap 30 on the potentiometer I1 is coupled through condenser I9 to the grid III of the discharge device. This grid III is also connected to the cathode through a resistance 20 which has a value dependent upon the discharge device I employed, such as to provide a suitable bias upon the grid of amplifier I produced by its own contact potential.

High frequency potentials appearing upon the anodes of discharge devices I and 2 are bypassed to the cathodes thereof, as is usual in such amplifiers, by condensers 2| and 22 respectively.

The discharge device 2 is preferably one having a screen grid I and a suppressor grid 32 and having extremely high impedance between its anode and cathode. A suitable bias voltage for the control grid of thisdischarge device is provided by means of resistance 23, which is by passed for signal frequencies by condenser 24. The grid of device 2 is connected to the negative terminal of this resistance 23 through the usual grid resistor 3|.

Condensers 25 and 26 are the usual smoothing condensers associated with the power supply system which is connected between the conductors +3 and -B.

In the operation of my invention the intermediate frequency currents of the receiver, which commonly have a frequency of 455 kilocycles, are rectifier in the diode 9, I 2, the rectified potentials appearing upon resistance I4. The audio frequency components of these potentials are supplied across the potentiometer II from which they are coupled by condenser I8 to the grid of amplifier I. These components are amplified in amplifier I and are then supplied through coupling condenser 3 to the grid of amplifier 2 from which they are supplied through the output transformer 4 to the loud speaker, or other suitable audio frequency apparatus.

The anode of the amplifier 2, it will be observed, is connected to the power supply of the receiver at the input side of-its output filter,

which filter comprises resistance 8 and condensers and 26. Accordingly, considerable amounts of hum currents produced by rectification of the energizing currents, which commonly are of sixty-cycle frequency, are supplied to the anode circuit of the amplifier 2 and these hum currents are likely to be reproduced by the loud speaker. Of course, the application of such hum currents to the anode of the output tube could be reduced by additional filtering but such filtering is expensive and, in addition, it includes elements across which voltage drops appear, making it necessary to use higher voltage transformers in the power supply system. I

In accordance with my invention, however, the reproduction of these hum currents is obviated by the use of degeneration in an improved way. This is effected by'means of the resistances; I5 and I8 in the anode circuit of discharge device 2,

' across which the diode load I4 and the volume control potentiometer I! are connected in series. These elements I 4, I1, I8 and I5 comprise a bridge and may be so proportioned that the audio frequency voltage with respect to ground, derived from passage of anode current of device 2 through resistances I5 and I8, is substantially the same at the point 21 on the branch I 4, I1 of the bridge as at the point 28 on the branch I5, I8 of the bridge. This may be so even at low frequencies-since the coupling condenser I6 may be of very high capacity and low impedance.

It will be observed, therefore, that when the variable contact 30 of the potentiometer is at its upper position, substantially no voltage from resistances I8 and I5 is supplied to the input of the discharge device I. That is, the voltage of resistances I8 and I5 areboth supplied to this point, the voltage of resistance I8 being supplied in degenerative phase and the voltage of resistance I5 being supplied in regenerative phase with respect to the input of discharge device I, so that the two substantially neutralize each other and no degeneration is produced. Also, at this point the maximum voltage of the source comprising resistance I4 is supplied to the grid of the amplifier. This is the position which the potentiom-' eter would occupy during reception of weak signals as from distant stations.

When the potentiometer contact 30 is moved to its lower position the entire voltage of resistance I8 is supplied to the input of the amplifier. This appears from the fact that the cathode is connected to the point 28 between resistances I5 and I8 and the grid is connected to the upper terminal of resistance I8.. Accordingly this voltage is efiective upon the grid of the amplifier and 2I and 22, and at low frequencies by reason of the I coupling condenser 3 and the inductance of the output transformer 4, so that the feed back volt- I age which is commonly taken from across the output device, and supplied back to the grid becomes so shifted that it becomes regenerative. Such regeneration at high frequencies tends to cause increased amplification at such frequencies, which tends, in turn, to accentuate the reception of noise currents, etc. Such regeneration at low. frequencies tends to increase the amplification at low frequencies and, hence, accentuates the undesired hum currents in the output of the system. In short wave radio receivers, this increased low frequency amplification increases the tendency of the receiver to produce howling and fiuttering sounds at the reproducer. In fact, such regeneration at both high and low frequencies may be so great as actually to produce continuous oscillations.

To reduce such undesired regeneration the amount of degeneration employed may be reduced. This is undesirable, however, since this increases the production of hum and, in general, reduces the advantages secured through degeneration, such as reduction of distortion, improvement of the frequency characteristic, etc.

In accordance with my invention, increased degeneration may be employed and at the same time this undesired regeneration may be com- .pletely avoided.

This resultis effected by the arrangement described in that the resistances I5 and I8 from which the feed back voltages are derived are connected in series with the output transformer I and in that the discharge device 2 is of very high impedance. This discharge device is one in which the internal anode impedance is so high that it is commonly referred to as a constant current device. That is, the phase of the anode current is not affected at the different frequencies by rea son of the reactance of the output transformer or other reactances which may be included in the anode circuit. Since this current is passed through the resistances I5 and I8 from which the feedback voltages are derived, it will be seen that the phase of these voltages are not affected at the different frequencies by the reactance of the output transformer. Thus, no regeneration can be produced by reason of the impedances of the anode circuit of discharge device 2.

The only remaining impedances which might produce phase shift and therefore, regeneration, are condenser 2| and condenser 3 which are included in circuit between the discharge devices. The capacitance of condenser 3 may readily be made sufficiently large and that of condenser 2I sufiiciently small, however, with respect to resistor 3I that any phase shift produced by these elements is appreciable only at very low frequencies or at frequencies above the audio range respectively, and even at those frequencies the shift cannot exceed ninety degrees and accordingly the system cannot become regenerative. Accordingly, the only effect of these impedances is to re duce the extent of degeneration, and this reduction, by proper choice of these impedances, may be made small in the usable frequency range.

Condenser 22 may be connected between the anode of discharge device 2 and the point 28 of the resistances I I and 18 so that at the high frequencies more audio voltage appears across resistance I 8 than appears across resistance I5. In this way, more degenerative voltage is supplied back to the input circuit at high frequency than regenerative voltage, with the result that when the potentiometer 30 is near the upper end of the potentiometer resistance II, the system is balanced at medium frequencies but is still degenerative at high frequencies so that noise on weak signals is reduced.

Of course, when the potentiometer contact 30 is at the top of the volume control, unless something further were done, the low frequency degeneration voltage would be completely removed. It, therefore, would not be effective to reduce hum, or the tendency above mentioned to produce howl, or fiutter, of, the receiver. Accordingly, it is desirable that there be some degeneration at-low frequencies when the potentiometer is adjusted at this upper position. This may be effected by reducing the capacity of condenser I6 so that the bridge l1, l4, l8, I5 is somewhat unbalanced at low frequencies.

of course, it is undesirable to make the ca- Dacitance l6 too small because to do so would reduce the desired transm ssion of low and medium frequency potentials from res stance 14 to potentiometer l'l. However, resistance I! may generally be of about 2 megohms resistance and about ten times greater than the resistance of resistor Id. With these elements so proportioned. the capacitance of condenser 16 may be made sufficiently small to produce sufiicient unbalance of the bridge at low frequencies to produce the desired degeneration and, at the same time, not materially reduce the desired transmission of low and medium frequency potentials from resistance l4 to potentiometer ll. That is, the reactance of condenser l6, when adjusted for desired degeneration with contact 30 at the top of the potentiometer I! is relatively small as compared to resistance I! so that it does not substantially reduce the current produced in resistance H by the potential of resistance M. It is, however, sufficiently large as compared to the resistance H to be effective in unbalancing the bridge with respect to potentials produced by the anode curent of device 2 flowing in resistances l5 and I 8.

One way to adjust condenser I6 is by first inserting a large condenser of, let us say, 1 microfarad in this position and then establish a true balance of the bridge at 400 cycles frequency. After this, contact 30 may be adjusted for good volume of reception from a semi-local station, and the capacity of condenser I 6 reduced until the low frequency response of the amplifier is reduced just noticeably. It will then be found that with contact 30 at the top of the volume control there will be sufficient degeneration effectively to reduce hum.

In the drawing, the potentiometer I1 is shown as having its lower portion shunted by the series combination of resistance 33 and capacitance 34 thereby partially to shunt out the medium and high frequencies for so-called base compensation. These elements, of course, influence the value of condenser I6 necessary for desired degeneration with contact 30 at the top of the volume control resistance H and if such elements be employed, the value of condenser I6 must be determined with them included in the circuit.

Of course, the balance of the bridge may, as is sometimes desirable, be so adjusted that when contact 30 is at the top of the volume control a regenerative voltage is supplied to the grid, thereby to increase amplification and sensitivity of the receiver. This may be eflected by proper choice of resistances ll, l1, l5 and I! so that the point of balance falls on potentiometer ll slightly below the upper terminal of resistance l1, rather than at its upper terminal for medium and high frequencies. Condenser l6 may be sufficiently small so that this regeneration does not exist for low frequencies and, in fact, so that for such frequencies, the system may even be degenerative when contact 30 is at, or near its extreme upper position. In this way advantage of regeneration may be had to increase the sensitivity of the receiver to middle frequencies such as 400 cycles and high frequencies, and, at the same time, hum and the tendency to produce howl and flutter are reduced by degeneration at low frequencies, all with contact 30 at the saine position, high on the volume control potentiometer.

It will be noticed that capacitance 14', which is the conventional intermediate frequency bypass condenser for the diode load resistance H, has its lower terminal connected between resistance l8 and I5 rather than directly to the lower terminal of resistance It. This is to avoid unbalance of the bridge at high frequencies which would result if this condenser were included in an arm thereof rather than across the diagonal 21, 28 thereof as shown.

Resistance I5 is included in circuit between the cathode and anode of diode 9, l2 but it is sufiiciently small in value so that the unidirectional voltage across it does not objectionably affect the operation of the diode. Ordinarily it may be of the order of 10 ohms.

While I have shown a particular embodiment of my invention it will be understood that I do not wish to be limited thereto, since different modifications, both in the circuit arrangements and in the instrumentality employed may be made. I, therefore, contemplate by the appended claims '.to cover any such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination, in an amplifier, of means to supply signals to the input circuit of said amplifier having frequencies extending over a range of signal frequencies, an output circuit for said amplifier including a reactive impedance, the phase characteristics of said impedance and said amplifier being such that if the voltages across said impedance were supplied to said input circuit in proper phase to produce degeneration at low frequencies within said range then undesirable regeneration would occur at high frequencies within said range due to variations in said characteristics with frequency, means to maintain the phase of the currents in said output circuit substantially unaffected by said variations, means responsive to said currents to supply potentials from said output circuit to said input circuit in degenerative phase at all frequencies within said range, and means to vary simultaneously the intensity of said signals supplied to said input circuit and the intensity of said potentials supplied thereto from said output circuit.

2. In combination, a source of audio signals, a signal load, an amplifier connected between said source and load, an impedance in series with said lead, saidsignal source and a second impedance connected in series across said first impedance, the input of said amplifier being connected between an intermediate point on said first impedance and a. point on said second imbeing sufiiciently great that the phase of current in said output circuit is substantially un-' afiected at different frequencies by the external impedance of said output circuit, said source and a second impedance being connected in series across said first impedance, and means for impressing upon said input circuit voltages develsistance' comprising the respective armsof a four-arm bridge, the voltage of said source and the voltage of said first resistance appearing on.

said second resistance, the input to said amplifier being connected to said second resistance, to receive therefrom both of said voltages, whereby the voltage of one portion of said first resistance tends to produce regeneration of said amplifier while the voltage of the other portion of said first resistance tends to. produce degeneration, and'means to supply a greater proportion of the output current of said amplifier through said I other portion of said first. resistance at high freoped between an intermediate point on said first impedance and a point on said second impedance, whereby said source and said first and second impedances form a .bridge connected to supply voltage from said output circuit to said input circuit. 1

4. In combination, an amplifier operable over a range of frequencies and having an output circuit having reactive impedance such that the phase of the voltage across said impedance varies with frequency, means to supply from said output circuit to the input of said amplifier, in.

degenerative phase, a, voltage having a phase substantially unaffected at the difierent fre- .quencies in said range by said reactive impedance, a source of signal currents to be amplified connected to said input, and means to vary simultaneously the amount of signal voltage supplied to said input and the amount of degeneratentiometer and the other input terminal being connected to an intermediate point on said resistance.

6. In an electron discharge amplifier having,

a pair of input terminals and a pair of output terminals, a load device and a resistance connected in series between said output terminals, 9. source of signal currents to be amplified and a potentiometer connected in series across said resistance, one input terminal of said amplifier being connected to a variable point on said potentiometer and the other input terminal being connected to an intermediate point on said resistance, said potentiometer and said signal source having such proportions that when said variable point is moved to one extremity of said potentiometer, substantially no voltage from said resistance is supplied to said input terminals.

7. In combination, a source of signal currents, a load, an amplifier connected between said source and load, a resistance in series with said load, having an intermediate point, the portions of said resistance on opposite sides of said intermediate point, said source, and a second requencies than at low frequencies thereby to increase the degeneration of said amplifier at high frequencies. i 1

8. In combination, an amplifier having a load and an impedance in series therewith, a diode detector having a load resistance and a load condenser, one terminal of said load resistance being connected to one terminal of said impedance and the other terminal of said load resistance being connected to an intermediatepoint on said impedance through said condenser, the opposite terminal of said impedance being con-, nected to said other terminal of said load resistance through a second resistance, and means to supply potential from said second resistance to the input of said amplifier, said load resistance, impedance and second resistance comprising a bridge having said condenser diagonal thereto.

9. In combination, an amplifier operating over a wide band of audio frequencies including a predetermined frequency of the order of to 400 cycles per second, a feed-back network connected fromthe output of said amplifier to the input thereof, and means to supply feedback voltages through said network'which are in regenerativephase at all frequencies within said band higher than said predetermined frequency and in degenerative phase at all frequencies within said band lower'than' said predetermined frequency, saidv voltages being of sufiicient magnitudes to increase the sensitivity of said amplifier at said higher frequencies and to decrease the tendency of said amplifier to reproduce hum voltages and to flutter at said lower frequencies.

10. In combination, an amplifier operating over a wide band of signalfrequencies extending on either side of a predetermined frequency, means comprising a bridge network interconnecting the output and input circuits of said amplifier for feeding back voltages of said frequencies, said network being balanced at said predetermined frequency and so proportioned that the feedback voltages impressed on said input circuit are regenerative at all frequencies within said band higher than said predetermined frequency and degenerative at all frequencies within said band lower than said predetermined frequency.

a 11. In combination, an amplifier operating over a wide range of signal frequencies, a volume I phase for one adjustment of said volume control device and in regenerative phase for a different adjustment of said volume control device.

12. In an electron discharge amplifier having a pair of input terminals and a pair of output terminals, a load device and a resistance connected in series between said output terminals,

a source of signal currents to be amplified and a potentiometer connected in series-across said resistance, one input terminal of said amplifier being connected to a variable point on said potentiometer and the other input terminal being connected to an intermediate point on said resistance, said source, potentiometer and resistance forming a bridge to supply voltage from 'the circuit of said load device to the input of said amplifier, and means so to balance said bridge that when said variable point is in one position on said potentiometer high frequency currents are supplied to said input in regenerative phase and low frequency currents are supplied thereto in degenerative phase while, when said variable point is at a different position on said potentiometer both low and high frequency currents are supplied in degenerative phase.

13. The combination, in an audio amplifier including a plurality'ofelectron discharge devices connected in cascade, of a power supply rectifier having an output smoothing filter, the anode of the last discharge devic'e'in said cascade connection being connected for unidirectional current through a reactive output device to the input of said filter whereby objectionable amounts of hum currents are supplied to said last discharge device, a resistance connected between the cathode of said last discharge'device and said rectifier, and means to supply signal and hum voltage from said resistance to the input of the first discharge device in said cascade connection in degenerative phase, thereby to reduce the hum currents appearing in said out- I put device, said last discharge device having such high impedance that the phase of voltage on said resistance is substantially unaffected by the last discharge device in said amplifier, reactance in circuit between said stages, a source of operating potential for the last of said stages producing undesired low frequency variations insaid output circuit, and means to reduce said variations, said means comprising a resistance,

in series with said output circuit, means to supply signal variations and said undesired low frequency variations to the input of the first of said amplifiers in degenerative phase, the anode to cathode impedance of said last discharge device being sufficiently high to prevent the phase of voltage on said resistance from being materially affected by the reactance of said output circuit, whereby the phase shift is substantially limited to that caused by reactance between said stages, the phase shift produced by reactance between said stages being limited to less than ninety degrees at all frequencies at which said amplifier operates, whereby said interstage and output circuit reactances never render said amplifier regenerative.

' CHARLES S.'ROOT. 

